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Good point. The docs don't actually say, except that the docs show a pretty elaborate process dance, where the source process only closes its side of the socket once the target process has called WSASocket().
If duplication happened early, then there would have to be a way to "unduplicate" it in the source process if, say, IPC somehow failed. There is currently no api to undo the effects of WSADuplicateSocket().
It's easy enough to test, of course. I ́ll do some test using my unittest.